The performance of multimode fiber (MMF) is largely governed by the amount of dispersion a pulse undergoes as it propagates through the fiber. Dispersion is the broadening of discrete data bits or “symbols” as the bits propagate through a media. Dispersion results in an overlap between adjacent data bits causing an increase in the uncertainty that a bit will be interpreted as a logic 0 or 1. This uncertainty in logic state can be quantified in terms of bit error rate (BER), where the BER is defined as the number of bit errors divided by the total number of bits transmitted in a given period of time. For high-speed Ethernet, the BER cannot exceed 1 error bit for every 1 trillion bits transmitted (BER<10−12). Modal dispersion results from the difference in propagation velocities between the various modes traversing the optical fiber. Since the optical power is carried by the sum of the discrete modes, as the modes spread in time the optical power of the pulse disperses. Modal dispersion is expressed in terms of Differential Mode Delay (DMD), which is a measure of the difference in pulse delay (ps/m) between the fastest and slowest modes traversing the fiber.
The index of refraction of a material represents the amount by which the speed of light is reduced within the material, as compared to the speed of light in a vacuum. Since the refractive index of a material, normally given the abbreviation “n,” is wavelength-dependent (that is, the index is a function of wavelength, which can be written as “n(λ)) the velocity of light in a material is also wavelength-dependent, and the velocity as a function of wavelength is related to the wavelength dependence of the index of refraction by,
      v    ⁡          (      λ      )        =      c          n      ⁡              (        λ        )            where c is the speed of light in vacuum (299,792,458 meters/second).
Hence, a pulse of light having a finite spectral width will also undergo wavelength dispersion as it propagates through a material. This is called chromatic dispersion. In multimode fiber, modal dispersion is typically much larger than chromatic dispersion, however in high-bandwidth MMF (>8000 MHz·km), chromatic dispersion begins to dominate. It follows that by reducing the dispersion in MMF, the performance of the fiber will increase.
Using a BER test bench, it has been discovered that current industry-standard fiber performance metrics do not accurately predict the fiber's system performance. In FIG. 1, the BER channel performance is shown as a function of the calculated Effective Modal Bandwidth (EMBc) for 81 300 m high-bandwidth MMF's. In order to compare bit error rates, we measure the BER at a reference optical power level of −11.0 dBm. It is important to note that the BER test bench employed in these tests simulates worst-case conditions for a 10 GBASE-SR Ethernet link. The calculated Effective Modal Bandwidth (EMBc) values were determined from DMD measurements and are related to the Effective Modal Bandwidth (EMB) by a factor of 1.13 (i.e., EMB=1.13×EMBc).
The data show a poor correlation between EMBc (or EMB) and BER. We find that for a nominal EMBc of 2000 MHz·km, the BER performance of a MMF can vary by more than 4 orders of magnitude. At the reference optical power level, a BER greater than 2E-08 is considered a channel failure. The data show that many fibers will not support 10 Gb/s Ethernet transmission to the specified maximum channel length of 300 m. Because most channel links in the data center do not approach the maximum reach limit, system failures have not been widely observed. However, several fiber-related channel failures have been reported.